1. Field of the Invention
The invention relates to a propeller shaft that includes an input shaft and an output shaft.
2. Description of Related Art
For example, Japanese Patent Application Publication No. 6-173934 (JP 6-173934 A) describes a conventional propeller shaft that includes an input shaft and an output shaft, and that is interposed between a transmission and a differential gear to transmit an output from the transmission to the differential gear.
The input shaft has a cylindrical portion, and is coupled to an output portion of the transmission via a first joint portion.
The output shaft is connected to the input shaft via a telescopic portion, and is also coupled to an input portion of the differential gear via a second joint portion. On the outer periphery of the output shaft, grease used as lubricating oil is applied and a bellow seal member (boots) that covers a part of the cylindrical portion of the input shaft is stretchably provided. With this configuration, the telescopic portion is sealed in the seal member, and therefore the sealing performance is improved.
The telescopic portion has a spline sliding mechanism that has an internal spline and an external spline (an inner periphery spline and an outer periphery spline), and is provided between the inner peripheral face of the cylindrical portion of the input shaft and the outer peripheral face of the output shaft.
With the configuration described above, as the input shaft and the output shaft move relative to each other via the telescopic portion in the axial direction of the propeller shaft, the propeller shaft telescopes accompanied with a stretch or contraction of the seal member.
It is widely known that, in this type of propeller shaft, an intermittent motion of the input shaft and the output shaft (stick-slip) occurs while the propeller shaft telescopes, if the gradient of a straight line indicating the μ-v characteristics is negative. The μ-v characteristics show the relationship between the friction coefficient (μ) and the slide speed (v) of the telescopic portion (between the inner periphery spline and the outer periphery spline).
Therefore, in order to suppress occurrence of stick-slip, it is important to set the gradient of the straight line indicating the μ-v characteristics to a positive gradient, in other words, to reduce an amount of decrease in the slide resistance with respect to an increase in the speed of slide between the inner periphery spline and the outer periphery spline.
The coefficient of friction (μ) between the inner periphery spline and the outer periphery spline is reduced using, for example, grease in order to suppress occurrence of stick-slip. However, the sufficient effect of suppressing occurrence of stick-slip is not obtained.
It is possible to suppress occurrence of stick-slip by performing a coating process with the use of a solid lubricating film, for example, Diamond Like Carbon (DLC). In this case, however, the cost increases.